The Hepatitis B virus (HBV) is a major health problem worldwide and more than 350 million people are chronic carriers. HBV infection is a serious and common infectious disease of the liver. Chronic infection is associated with an increased risk to develop severe liver diseases, including liver cirrhosis and hepatocellular carcinoma (HCC), one of the most common forms of human cancer. The estimated risk of HCC in chronic HBV carriers is approximately 100 times greater than in uninfected individuals. About a third of the world population has been infected at one point in their lives, including 240 million to 350 million who have chronic infections. Over 750,000 people die of hepatitis B each year. About 300,000 of these are due to liver cancer. Currently available anti-HBV drugs have limitations. For example, interferon alpha administration is associated with severe adverse reactions. Nucleoside analogues are virostatic and require long-term administration.
The HBV genome exhibits genetic variability with an estimated rate of 1.4-3.2×10−5 nucleotide substitutions per site per year. A large number of virus variants arise during replication as a result of nucleotide misincorporations in the absence of any proof reading capacity by the viral polymerase. This variability has resulted in well-recognized subtypes of the virus. HBV has been classified into well-defined genotypes on the basis of an inter-group divergence of 8% or more in the complete genomic sequence, each having a distinct geographical distribution. For example, Genotype A is widespread in sub-Saharan Africa, Northern Europe, and Western Africa; genotypes B and C are common in Asia; genotype C is primarily observed in Southeast Asia; genotype D is dominant in Africa, Europe, Mediterranean countries, and India; genotype G is reported in France, Germany, and the United States; and genotype H is commonly encountered in Central and South America. Genotype I has recently been reported in Vietnam and Laos. The newest HBV genotype, genotype J, has been identified in the Ryukyu Islands in Japan.
HBV is an enveloped DNA virus that belongs to the Hepadnaviridae family. It contains a small, partially double-stranded (DS), relaxed-circular DNA (rcDNA) genome that replicates by reverse transcription of an RNA intermediate, the pregenomic RNA (pgRNA). The circular DNA genome of HBV is unusual because the DNA is not fully double-stranded. One end of the full length strand is linked to the viral DNA polymerase. The genome is approximately 3020-3320 nucleotides long (for the full-length strand) and 1700-2800 nucleotides long (for the short length-strand). The negative-sense (non-coding) is complementary to the viral mRNA.
There are four known genes encoded by the genome, referred to as C, X, P, and S. The core protein is coded for by gene C (HBcAg), and its start codon is preceded by an upstream in-frame AUG start codon from which the pre-core protein is produced. The HBeAg is produced by proteolytic processing of the pre-core protein. The DNA polymerase is encoded by gene P. Gene S codes for the surface antigen (HBsAg). The HBsAg gene is one long open reading frame but contains three in frame “start” (ATG) codons that divide the gene into three sections: pre-S1, pre-S2, and S. Because of the multiple start codons, polypeptides of three different sizes called Large (the order from surface to the inside: pre-S 1/pre-S2/S), Middle (pre-S2/S), and Small (S) are produced. The function of the protein coded for by gene X is not fully understood but it is associated with the development of liver cancer. It stimulates genes that promote cell growth and inactivates growth regulating molecules.
The viral DNA is found in the nucleus soon after infection of the cell. The partially double-stranded DNA is rendered fully double-stranded by completion of the (+) sense strand and removal of a protein molecule from the (−) sense strand and a short sequence of RNA from the (+) sense strand. Non-coding bases are removed from the ends of the (−) sense strand and the ends are rejoined.
The HBV life cycle begins when the virus attaches to the host cell and is internalized. Recent studies have demonstrated that sodium-taurocholate co-transporting polypeptide (NTCP) is a functional receptor in HBV infection. The virion relaxed circular DNA (rcDNA) is delivered to the nucleus, where it is repaired to form a covalently closed-circular DNA (cccDNA). The episomal cccDNA serves as the template for the transcription of the pregenomic RNA (pgRNA) and the other viral mRNAs by the host RNA polymerase II. The transcripts are then exported to the cytoplasm, where translation of the viral proteins occurs. Reverse transcriptase (RT) binds to pgRNA and triggers assembly of the core proteins into immature, RNA-containing nucleocapsids. The immature nucleocapsids then undergo a process of maturation whereby pgRNA is reversed transcribed by RT to make the mature rcDNA. A unique feature of hepadnavirus reverse transcription is the RT primed initiation of minus-strand DNA synthesis, which leads to the covalent linkage of RT to the 5′ end of the minus-strand DNA.
The mature, rcDNA-containing nucleocapsids are then enveloped by the viral surface proteins and secreted as virions (secretion pathway) or, alternatively, are recycled back to the nucleus to further amplify the pool of cccDNA (recycling pathway). Persistence of cccDNA in hepatocytes plays a key role in viral persistence, reactivation of viral replication after cessation of antiviral therapy, and resistance to therapy.
Homing endonucleases are a group of naturally-occurring nucleases which recognize 15-40 base-pair cleavage sites commonly found in the genomes of plants and fungi. They are frequently associated with parasitic DNA elements, such as group 1 self-splicing introns and inteins. They naturally promote homologous recombination or gene insertion at specific locations in the host genome by producing a double-stranded break in the chromosome, which recruits the cellular DNA-repair machinery (Stoddard (2006), Q. Rev. Biophys. 38: 49-95). Homing endonucleases are commonly grouped into four families: the LAGLIDADG (SEQ ID NO: 2) family, the GIY-YIG family, the His-Cys box family and the HNH family. These families are characterized by structural motifs, which affect catalytic activity and recognition sequence. For instance, members of the LAGLIDADG (SEQ ID NO: 2) family are characterized by having either one or two copies of the conserved LAGLIDADG (SEQ ID NO: 2) motif (see Chevalier et al. (2001), Nucleic Acids Res. 29(18): 3757-3774). The LAGLIDADG (SEQ ID NO: 2) homing endonucleases with a single copy of the LAGLIDADG (SEQ ID NO: 2) motif form homodimers, whereas members with two copies of the LAGLIDADG (SEQ ID NO: 2) motif are found as monomers. Methods for producing homing endonucleases are known in the art.
I-CreI (SEQ ID NO: 1) is a member of the LAGLIDADG (SEQ ID NO: 2) family of homing endonucleases which recognizes and cuts a 22 basepair recognition sequence in the chloroplast chromosome of the algae Chlamydomonas reinhardtii. Genetic selection techniques have been used to modify the wild-type I-CreI cleavage site preference (Sussman et al. (2004), J. Mol. Biol. 342: 31-41; Chames et al. (2005), Nucleic Acids Res. 33: e178; Seligman et al. (2002), Nucleic Acids Res. 30: 3870-9, Arnould et al. (2006), J. Mol. Biol. 355: 443-58). Methods for rationally-designing mono-LAGLIDADG (SEQ ID NO: 2) homing endonucleases were described which are capable of comprehensively redesigning I-CreI and other homing endonucleases to target widely-divergent DNA sites, including sites in mammalian, yeast, plant, bacterial, and viral genomes (WO 2007/047859).
As first described in WO 2009/059195, I-CreI and its engineered derivatives are normally dimeric but can be fused into a single polypeptide using a short peptide linker that joins the C-terminus of a first subunit to the N-terminus of a second subunit (Li et al. (2009), Nucleic Acids Res. 37:1650-62; Grizot et al. (2009), Nucleic Acids Res. 37:5405-19). Thus, a functional “single-chain” meganuclease can be expressed from a single transcript.
The use of engineered meganucleases for treatment of HBV infections has been suggested. For example, WO2010/136841 suggests the use of engineered meganucleases for cleaving the genome of non-genomically integrating viruses. Such meganucleases include variants of I-CreI. The '841 application discloses a number of 22 base pair meganuclease recognition sequences present in the genome of one HBV strain. However, the '841 application does not identify any recognition sequences that are present within multiple genotypes of the HBV genome.